1. Field of the Invention
The present invention relates to an antenna device communicating with radio tags, a radio tag reader and an article management system. In particular, the present invention relates to an antenna device which is able, for example, to collectively read tag information from a plurality of closely located radio tags.
2. Related Art
Generally, patch antennas, for example, have been used as antennas for conventional radio tag readers, as disclosed, for example, in JP-A 2005-167416 (Kokai). In the case where a large number of radio tags are present, the radio tag reader described in this literature is adapted to bring these radio tags into alignment with each other and read the tags only through a portion of a radio emission area of an antenna, which portion has a predetermined power density. For example, radio tags are stuck onto envelopes and then the plurality of envelopes are put together in a storage box for bringing into alignment with each other in the storage box. The storage box is then set for the reader, so that the information recorded on the plurality of radio tags can be read by the reader through the portion of the predetermined power density of the antenna provided in the reader.
However, the prior art described in the literature mentioned above has tended to cause interference when a plurality of radio readers are simultaneously in operation, because radio waves are constantly emitted from the antennas of the individual readers. Also, being influenced by the reflected waves, some portions of the radio tags have been prevented from being read, or radio tags that are not required to be read have been read.